Magnets are surrounded by lines of magnetic force, which comprise their magnetic fields. The places where the field lines converge were first named as poles by French scholar, Petrus Peregrinus, in the year 1269, over 300 years before William Gilbert's publication of "De Magnete" that began the formal study of magnetism as a science. Each magnet has 2 magnetic poles, defined as a north pole (or north-seeking pole) and a south pole (or south-seeking pole). There are several ways to determine magnetic polarity, depending on the type of magnet; these ways are described below.

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Steps

Method 1 of 3: Determining the Polarity of a Bar Magnet

1

Tie a string around the center of the bar magnet.

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2

Suspend the magnet from the string so it can turn freely. You can hold the other end of the string in your hand or tie it to an overhanging object. Keep the magnet away from iron or other paramagnetic metals, such as nickel, so that neither of its poles is attracted to the metal.

3

Let the magnet come to rest in 1 direction. The magnet will align itself with Earth's north and south magnetic poles. The magnetic pole pointing toward the North Magnetic Pole is the magnet's north-seeking pole, or north pole, for short. The magnetic pole pointing toward the South Magnetic Pole is the magnet's south-seeking pole, or south pole, for short.

This property of magnetic attraction led to the development of the magnetic compass. Early compasses were pieces of magnetic iron (called "lodestone" for its north-seeking ability) suspended basins of water, while modern hikers' compasses use a magnetized strip suspended on a small shaft enclosed in a disc-shaped case.

Because opposing magnetic poles attract, this means that Earth's North Magnetic Pole is actually a south-seeking pole, while the South Magnetic Pole is actually a north-seeking pole.

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Method 2 of 3: Mapping a Bar Magnet's Field

1

Lay the bar magnet on a non-magnetic surface.

2

Place a piece of glass over the magnet.

3

Sprinkle iron filings over the glass surface. The bar magnet's field will magnetize each of the filings.

4

Tap the glass gently. The magnetized iron filings will form themselves into elliptical loops on either side of the bar magnet. The greatest concentrations of filings will be where the loops converge, at the magnet's north- and south-seeking poles.

Another way to map a bar magnet's field is to place the magnet on a piece of paper and use a magnetic compass to trace the field by placing it at a number of points and marking where the needle is drawn to one of the poles. When the points are connected, they will form the same concentric loops as the iron filings did.

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Method 3 of 3: Determining the Polarity of an Electromagnet

1

Wrap the fingers of your right hand around the electromagnet's coil in the direction the electric current is flowing. Electric current actually flows from the source's negative pole to its positive pole; however, for the purposes of this rule, we treat the current as though it were flowing from the positive to the negative pole.

To determine polarity using the actual direction current flows, use your left hand instead of your right.

2

Extend your thumb at a right angle to the rest of your fingers. The direction your thumb is pointing is the north-seeking pole of the electromagnet. Magnetic fields project outward from the magnet's north-seeking pole and are drawn inward to the south-seeking pole.

Magnetic fields are constant in direction only for direct current electric fields, such as those in batteries. Magnetic fields in alternating current sources, such as household current, change polarity every time the electric current changes direction; the more frequently the current changes direction, the faster the polarity changes. This aspect enables audio speakers to reproduce sounds of varying pitches; the faster the current changes direction, the faster the magnetic fields change polarity, driving the speaker cone in and out faster to produce a higher-pitched sound.

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Just as the magnetic field of an electromagnet connected to an alternating current source changes direction, so also does the magnetic field of Earth change direction over time. Several such reversals have been found in magma that had seeped out of mid-ocean ridges at the time crustal plates separated and cooled, its magnetic particles aligned with the prevailing magnetic field direction at the time.

Warnings

Earth's magnetic poles are not the same as its geographic poles. In 2001, the North Magnetic Pole was located near Ellesmere Island in northern Canada. As do the geographic poles, the magnetic poles move over time at varying directions and speeds; in 2009, the North Magnetic Pole was moving toward Siberia at a rate of 34 to 37 miles (55 to 60 km) per year. (Currently, magnetic and geographic north are aligned along a line running from the North Magnetic Pole through Lake Superior and the Gulf of Mexico.)

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